1. Field of the Invention
The present invention relates to an image forming apparatus for forming a toner image on a photoreceptor.
2. Description of the Related Art
As is well known, there are various kinds of image forming apparatuses, but among them, an electrophotographic image forming apparatus has an imaging unit configured to perform the following process. First, a photoreceptor having been charged is irradiated with light modulated by image data to form an electrostatic latent image on the surface of the photoreceptor. Next, a developer (i.e., toner) is supplied to the electrostatic latent image on the surface of the photoreceptor, and a toner image is formed on the surface of the photoreceptor. The toner image formed according to the above-mentioned process is transferred on a paper sheet. Then, a toner image on the paper sheet is fixed, and printed matter is completed. Further, after the toner image is transferred, a cleaning unit removes the toner remaining on the surface of the photoreceptor.
Repeated printing repeats formation of the toner image and cleaning, and a photosensitive layer on the surface of the photoreceptor is abraded. When a remaining film thickness of the photosensitive layer is less than a lower limit to form the image, the life of the photoreceptor ends. Therefore, conventionally, various methods for measuring the thickness of the photosensitive layer (i.e., film thickness) have been proposed (e.g., see JP 2003-287409 A).
As is well known, when the surface of a photoreceptor is cleaned, a portion on which a toner image is formed is abraded more than a portion on which the toner image is not formed (i.e., non-image portion). In repeated printing, image coverage and/or paper sheet size is generally different each time. As mentioned above, the photoreceptor comes to have an uneven film thickness thereon. Since charging bias voltage is adjusted based on the uneven film thickness, a portion having a large film thickness may have image defect due to charging failure. Such image defect may end the life of the photoreceptor, although the photoreceptor has a sufficient remaining film thickness.